1. Field of the Invention
This invention relates generally to torque sensors, and in particular to non-compliant torque sensors utilizing a magnetoelastic element and a non-contacting magnetometer for sensing magnetic field changes that correspond to changing torque values in a rotatable shaft.
2. Related Art
Sensing the torque of rotating shafts is desirable in many applications, such as determining steering wheel effort measurements in electronic power steering systems, determining transmission output torque for electronically controlled shifting, determining power tool output torque, and the like. Torque sensors have been produced in many varieties, and can be generally classified in two categories: “compliant” and “non-compliant.” In compliant torque sensors, a sensor is attached directly to an elastic beam section of a torque-producing shaft to measure the mechanical twist of the shaft. Often, in compliant sensors, the shaft twists up to about eight degrees over the full measurement range.
In one example of a compliant torque sensor, a strain gauge is used to measure perceivable twisting of the elastic beam section of the shaft. When torque is applied to the elastic beam section, the strain gauge is deflected, which causes a resistance change in the strain gauge. This change of resistance in the strain gauge indicates a change in torque. However, due to the rotating nature of the beam section to which the strain gauge is attached, connecting wires to the strain gauge for transmission of signals is impractical. Thus, torque sensors that use a strain gauge require a wireless transmitting device, such as a radio-frequency transmitter, to transmit resistance changes in the strain gauge to a receiver, which interprets these signals as torque values. Alternatively, a signal transference scheme utilizing slip rings, brushes and commutators could be used in a compliant torque sensing system. Other compliant torque sensors may include rotary encoders or potentiometers that are attached to the shaft to measure the mechanical twist of the shaft, and the mechanical twist is then converted to a torque value.
However, such compliant torque sensing systems present numerous problems. For instance, because the strain gauges are attached directly to an elastic beam, torque limiters must be included on the rotating shaft to protect the beam and strain gauges from being deflected beyond their elastic range. Unfortunately, such precautions inherently interfere with the transmission of energy through the shaft, and, in the instance of a steering wheel shaft, provide a “soft feel” to the user. Additionally, such torque sensors are of limited reliability due to the direct contact with the rotating shaft, and are very expensive. Torque sensors that use a strain gauge also require frequent calibration.
To overcome these problems, non-compliant torque sensors have been developed, whereby a sensor monitors shaft torque changes in a non-contacting manner, thus obviating the need for torque limiters. Normally, such torque sensors utilize a magnetoelastic element intimately attached to a rotating shaft, whereby the torque sensor operates on the principle of inverse-magnetostriction.
Magnetostriction is well known in the art and describes a structural property of matter that defines a material's dimensional changes as a result of a changing magnetic field. In essence, magnetostriction is caused when the atoms that constitute a material reorient to align their magnetic moments with an external magnetic field. This effect is quantified for a specific material by its saturation magnetostriction constant, which is a value that describes a material's maximum change per unit length.
In contrast, inverse-magnetostriction defines changes in a material's magnetic properties in response to applied mechanical forces. Torque sensors that utilize inverse-magnetostriction operate on the premise that stresses and strains that are transmitted through the rotating shaft to the magnetoelastic element by the application of torque cause measurable changes in the magnetic field of the magnetoelastic element. Thus, the magnetic field strength produced from the magnetoelastic element is a direct function of the magnitude of the torque applied. A torque sensor utilizing such a magnetoelastic element also has a magnetometer that translates the magnetic field strength emanating from the magnetoelastic element into an analog voltage signal, thereby performing a torque-to-voltage transducer function.
It is known in non-compliant torque sensors to attach a ring of magnetoelastic material to a rotating shaft using interference fitting, such as a pressure fit or shrink fit, using an inter-engaging mechanism such as mating splines or teeth, using a chemical such as an adhesive, using a thermal connection such as thermal spraying, or any other method of attaching that is known in the art. In practice, under any of the above attaching methods, the attachment of the magnetoelastic element to the shaft has proven to be of the utmost importance. Indeed, defects in the boundary between the magnetoelastic element and the torque carrying member will result in aberrant coupling of stress and strains into the magnetic element, which adversely affect torque measurements. Boundary defects can include imperfections such as voids, contaminates, and lateral shearing.
It is also known to endow the magnetoelastic element with the magnetic attributes required by imparting the element with circumferentially directed stress (also known as hoop stress). The present state of the art uses tensile stress to achieve hoop stress. Tensile stress in the magnetoelastic element acts to stretch the material, which can result in increased porosity. The increased porosity advances a phenomenon known as corrosion cracking, which is the propagation of microscopic fissures in the structure. As a result of corrosion cracking, the material can eventually lose its tensile stress component causing a degradation of its magnetic properties, possibly even causing a complete failure of the sensor.
Further, practical requirements for torque sensors include design tolerance limits on the accuracy and linearity of the in-range voltage output and the amount of hysteresis, also known as “zero shift,” after a “yield torque” or “over-torque” is applied to the shaft. Such “over-torque” conditions can exist, for example, in steering systems during curb push-away situations, and can be experienced in transmission applications during drastic torque reversals. Hysteresis may occur because after the over-torque condition is relaxed, the resulting breakdown or slippage at the shaft/magnetoelastic element interface causes a mechanical bias in the magnetoelastic element. Consequently, a corresponding magnetic bias is produced, thereby negatively affecting future torque measurements.
There are a number of potential causes for the formation of the magnetic bias that affects future torque measurements and causes zero shift. For example, if the underlying shaft yields upon the application of a large torque, the shaft will not return to its quiescent state, and the measurement consequently will not return to zero. Alternatively, if a large torque causes the magnetoelastic element to yield, but does not cause the shaft to yield, then the magnetoelastic element will be stressed in the opposite direction when the torque is removed. Thus, the sensor will return to a position past zero, and the zero-shift in such a situation is negative. In sum, a zero shift in torque sensors can occur as a result of any of a combination of factors.
Further, if the breakdown of the shaft/magnetoelastic element interface is localized, the result may be a magnetic incongruity that manifests as a variance in torque measurements with respect to the angular position of the shaft. While such breakdown between the shaft and magnetoelastic element is normally not a problem where the magnetoelastic element is thermally sprayed, hysteresis still occurs in thermal sprayed magnetoelastic elements due to the different coefficients of thermal expansion between the shaft and magnetoelastic element, as will be explained in more detail herein.
For example, in an automotive steering column torque sensor, it is preferred that there be a full range torque measurement of +/−6 ft-lb, and a hysteresis requirement +/−1.5% of full scale after application of a 100 ft-lb yield torque. However, currently used thermal sprayed magnetic elements will exhibit hysteresis well over the acceptable limits even when a yield torque of only 15 ft-lb. is applied.
Thus, there is a need for a torque sensor that will exhibit low hysteresis after a yield torque is applied. Further, there is a need for a method of producing such a low hysteresis torque sensor.
Other needs will become apparent upon a further reading of the following detailed description taken in conjunction with the drawings.